1. Field of the Invention
The present invention relates to an ink jet head to eject ink onto a print medium for printing according to an ink jet method and also relates to a circuit board for the head, a method and a device for cleaning the head and an ink jet printing apparatus using the head.
2. Description of the Related Art
An ink jet printing method disclosed in U.S. Pat. No. 4,723,129 or U.S. Pat. No. 4,740,796 can perform a high-speed, high-quality printing by generating a bubble in ink using a thermal energy and can easily be upgraded to have a color printing capability and reduced in size. Because of these advantages, this method has become a mainstream of the ink jet printing method in recent years.
A general construction of the head (ink jet head) used for the ink jet printing comprises a plurality of ink ejection orifices, a plurality of liquid paths communicating to the ink ejection orifices, and a plurality of electrothermal transducers to generate a thermal energy to eject ink from the nozzles. The electrothermal transducer is constructed of a heating resistor and an electrode to supply electricity to the resistor. The electrothermal transducer is covered with an electrically insulating protective layer to secure insulation between the electrothermal transducers. Each ink path communicates with a common liquid chamber which is supplied ink from an ink tank containing ink. The ink supplied to the common liquid chamber is introduced into each liquid path and, near an ink ejection orifice, forms a meniscus which is kept there. In this state, when the electrothermal transducers are selectively driven, they generate a thermal energy which rapidly heats the ink through an ink contact member (heat application portion) situated immediately above the electrothermal transducer, generating a bubble in ink. A pressure of the expanding bubble ejects an ink droplet.
The heat application portions of such an ink jet head (hereinafter simply referred to also as a head) are each exposed to high temperatures due to the heat of the heating resistor and also subjected to combined influences including physical influences such as impacts of cavitations generated by expansion and contraction of the bubble and to chemical influences of ink. To protect the electrothermal transducer against these influences, the heat application portion is covered with a top protective layer. Conventionally, a protective layer of Ta, which has a relatively strong resistance against impacts of cavitations and chemical actions of ink, has been formed to a thickness of 0.2-0.5 μm to prolong the life of the head and enhance its reliability.
FIG. 26 is a schematic cross-sectional view showing a heat application portion and its surrounding portion of the conventional ink jet head. In FIG. 26, denoted 601 is a silicon substrate, 602 is a heat accumulating layer formed of a thermally oxidized film, SiO film or SiN film, 604 is a heating resistor layer, and 605 is an electrode wiring layer 605 for wires formed of such metal materials as Al, Al—Si and Al—Cu. A heating portion 604′ as the electrothermal transducer is formed by removing a part of the electrode wiring layer 605 to expose the corresponding part of the heating resistor layer 604. The heating resistor layer 604 is wired over the substrate 601 and connected to a drive element circuit or an external power supply terminal. With this arrangement, the heating resistor layer 604 can be supplied electricity from outside.
Designated 606 is a protective layer provided over the heating portion 604′ and the electrode wiring layer 605. The protective layer 606 also serves as an insulation layer made of a SiO film or SiN film. A reference number 607 represents an upper protective layer over the protective layer 606. The upper protective layer 607 protects the electrothermal transducer against the chemical and physical influences. A part of the upper protective layer 607 situated over the heating portion 604′ is the heat application portion that is in contact with and applies heat to the ink. The upper protective layer 607 is provided solely to protect the electrothermal transducer from chemical and physical impacts and is not electrically connected with external electrodes.
The ink jet head circuit board 600 of the above construction has a flow path forming member 620. The flow path forming member 620 has an ink ejection orifice 621 formed at a position corresponding to the heat application portion, and also a flow path formed therein which communicates from an ink supply port, that pierces the circuit board 600, through the heat application portion 608 to the ink ejection orifice 621.
In the heat application portion 608 of the ink jet head, colorants and additives, when heated to high temperatures, are resolved at a molecule level and turn into substances that are difficult to dissolve. These substances are adsorbed to the upper protective layer 607. This phenomenon is called a “kogation”. When hard-to-dissolve organic or inorganic substances adsorb to the upper protective layer 607, heat transmission from the heat application portion 608 to the ink becomes ununiform making the bubble generation unstable.
To minimize this kogation phenomenon a conventional practice involves using an ink containing a highly heat-resistant dye or an ink thoroughly refined to reduce the quantity of impurities in the dye. This, however, gives rise to other problems, such as an increased cost of ink or a limited number of kinds of dyes that can be used.
To solve these problems, Japanese Patent Application Laid-open No. 9-29985 (1997) discloses a cleaning method which fills the head with a water solution containing an electrolyte (kogation removing liquid), different from the ink, and applies electricity to the surface layer of Ta, which acts as heat application portion, to remove kogations accumulated on the heat application portion. In this cited document it is described that the application of electricity causes an electrochemical reaction between Ta and the water solution, which results in a part of the Ta layer surface being corroded and dissolved in the water solution to remove the deposited kogations along with the delaminating Ta layer.
For a stable generation of bubble in ink, it is important that the kogations deposited on the heat application portion be removed uniformly and reliably. However, an examination of the technique described in Japanese Patent Application Laid-open No. 9-29985 (1997) by the inventors of this invention have found a problem that the deposited kogations sometimes fail to be removed sufficiently. A further examination has revealed that the heating forms an oxide film over the surface of the Ta layer used as the upper protective layer and that this oxide film hinders the electrochemical reaction for removing kogations. That is, since the electrochemical reaction is hindered over the surface of the heat application portion where kogations are deposited, the kogations cannot be removed uniformly and reliably.
In Japanese Patent Application Laid-open No. 9-29985 (1997), a dedicated kogation removing liquid is used and needs to be supplied to the head before the cleaning is executed. This operation is performed either by a recycling company or by a user. There is, however, a problem that the cleaning cannot be done at least during the printing operation performed by the user.